In typical commercial reproduction apparatus (electrographic copier/duplicators, printers, or the like), a latent image charge pattern is formed on a uniformly charged charge-retentive or photoconductive member having dielectric characteristics (hereinafter referred to as the dielectric support member). Pigmented marking particles are attracted to the latent image charge pattern to develop such image on the dielectric support member. A receiver member, such as a sheet of paper, transparency or other medium, is then brought directly, or indirectly via an intermediate transfer member, into contact with the dielectric support member, and an electric field is applied to transfer the marking particle developed image to the receiver member from the dielectric support member. After transfer, the receiver member bearing the transferred image is transported away from the dielectric support member, and the image is fixed (fused) to the receiver member by heat and/or pressure to form a permanent reproduction thereon.
A reproduction apparatus generally is designed to generate a specific number of prints per minute. For example, a printer may be able to generate 150 single-sided pages per minute (ppm) or approximately 75 double-sided pages per minute with an appropriate duplexing technology. Small upgrades in system throughput may be achievable in robust printing systems, however, the doubling of throughput speed is mainly unachievable without a) purchasing a second reproduction apparatus with throughput identical to the first so that the two machines may be run in parallel, or without b) replacing the first reproduction apparatus with a radically redesigned print engine having double the speed. Both options are very expensive and often with regard to option (b), not possible.
Another option for increasing reproduction apparatus throughput is to utilize a second print engine in series with a first print engine. For example, U.S. Pat. No. 7,245,856 discloses a tandem printing system which is configured to reduce image registration errors between a first side image formed by a first print engine and a second side image formed by a second print image. Each of the '856 print engines has a photoconductive belt having a seam. The seams of the photoconductive belt in each print engine are synchronized by tracking a phase difference between seam signals from both belts. Synchronization of a slave print engine to a main print engine occurs once per revolution of the belts, as triggered by a belt seam signal, and the velocity of the slave photoconductor and the velocity of an imager motor and polygon assembly are updated to match the velocity of the master photoconductor. Unfortunately, such a system tends to be susceptible to increasing registration errors during each successive image frame during the photoconductor revolution. Furthermore, given the large inertia of the high-speed rotating polygon assembly, it is difficult to make significant adjustments to the velocity of the polygon assembly in the relatively short time frame of a single photoconductor revolution. This can limit the response of the '856 system on a per revolution basis, and make it even more difficult, if not impossible, to adjust on a more frequent basis.
Therefore, there is a continuing need for a less expensive, yet reliable, method for enabling a user of a reproduction apparatus to increase duplex throughput while enabling tighter control over print engine synchronization. There is also a need for a method for increasing productivity of the reproduction apparatus when switching back and forth between the duplex mode and a simplex mode.